PWM Fan connections for Duet 3 6XD / 6HC / 3HC

Marius Breuer

Hi all,

admittedly, i haven't kept up to date with the forum, but as far as I can tell this issue hasn't yet been answered/solved.
I'm trying to run Noctua fans on an upcoming 3d printer using a 6XD + 3HC setup, however this equally applies to the 6HC.
As per their whitepaper all Noctua fans require the PWM pin to operate at a 5V level.

The convenient 4 pin connectors can't be used because the fan supply voltage is either +12V (internal regulator) or V_FUSED (in my case 24V). Either way the PWM pin won't operate at 5V.

Using the 5-Pin IO Modules isn't any more convenient either, as they seem to supply io[x].out with 3.3V

Additionally i couldn't find the tacho documentation (internal pull up to 3.3V? 5V? 12V? V_FUSED?) and would like to confirm, that the current and voltage is meeting the Noctua spec:

I found that deviating from their requirement with Vcc 5V for their 12V fans seems to work, as long as the current limits are observed. I have used a diode (seen in red) on a custom PCB, as there have been reports of some fan variants having an internal pull-up to ie 12V on the fan side.

I guess the best way forward for me is to consider a custom PCB to switch the +12V PWM from the fan header to +5V and keep the other pins unchanged.

As far as I can tell 5V PWM is the default for PC-fans, so having dedicated fan connectors with this one flaw is very unfortunate.

Best,

Marius

dc42

@Marius-Breuer the Intel PWM fan specification requires that the fan includes an internal pullup resistor on the PWM input. See page 9 of https://www.intel.com/content/dam/support/us/en/documents/intel-nuc/intel-4wire-pwm-fans-specs.pdf. If the Noctua fans meet this specification then they can be connected directly to the 4-pin fan headers, with the voltage selector on those header set to either VIN (with VIN=24V) or 12V as required by the fan.

RogerPodacter

@Marius-Breuer i run a 12 volt Noctua fan connected directly to the 4 pin with no issues on a 3HC and a Duet 3 Mini 5+.

I dont recall that i had to change any wiring to make things work.

My fan is the Noctua NF-F12 industrialPPC-2000 PWM.

Marius Breuer

@dc42 thanks, that's great to know. Assuming i want to use those 4-Pin IO Headers for other stuff, the PWM pin is therefore switching between a direct connection to ground and being left unconnected or does it use a pull-up? If so, to which voltage and with which resistor?

Thanks so much for the quick help!

Best,

Marius

dc42

@Marius-Breuer the OUT_NEG pin on the Duet 4-pin OUT connector is switched to ground. There is no pullup resistor on the Duet connected to that pin. This makes it suitable to connect to the PWM pin of a 4-wire fan or to the negative terminal of a 2-wire fan.

achrn

@Marius-Breuer note further on in the Noctua white paper - "External pull-up is not necessary as the signal is pulled up to 3,3V/5V inside the fan."

However, Noctua also deprecates using a single transistor and relying on the pullup and favours a two transistor drive circuit for the PWM (also further on in the white paper). I have found with some fans a need to add an additional external pullup, and actually I've generally pulled up to 12V (though not with a Noctua fan, and I note they say don't pull to 12V) see e.g. https://forum.duet3d.com/post/306593.